Sorena S. Sorensen

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

The abundance, P-T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600°C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high-T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of “average” island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. “Exotic” LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by “subduction-zone metasomatism,” although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments.

Two high-pressure–low-temperature serpentinite-matrix mélange belts, Motagua fault zone, Guatemala: A record of Aptian and Maastrichtian collisions

Left-lateral motion along the North American-Caribbean plate boundary has juxta- posed two high-pressure-low-temperature (HP-LT) belts from separate Cretaceous colli- sions. These two belts have quite different ages and different suites of high-pressure as- semblages, yet they both contain jadeitite, a relatively rare rock type. This part of the plate boundary zone follows the Motagua River Valley in Guatemala, where it separates the Maya block (North American plate) from the Chortis block (Caribbean plate). On both sides of the bounding Motagua fault, tectonic slices of serpentinite-matrix melange host the HP-LT rocks. South of the fault, the melange slices contain eclogite, lawsonite eclogite, glaucophane eclogite, and blueschist blocks. North of the fault, the melange slices contain omphacite metabasite, albitite, and garnet amphibolite blocks, but lack intact eclogite. In addition to the dissimilar rock assemblages, 40 Ar/ 39 Ar geochronology of phen- gitic micas yields 77-65 Ma for northern and 125-113 Ma for southern blocks. These data suggest that the southern belt formed during Early Cretaceous (Aptian), northeastward- dipping subduction of the Farallon plate and collision of the Chortis block with western Mexico. The block was then displaced southeastward along this suture. In contrast, the northern belt records subduction related to the Maastrichtian collision of an extension of the Chortis block, perhaps the Nicaraguan Rise, with the Maya block.

Metasomatism and partial melting in a subduction complex Catalina Schist, southern California

Partial melting accompanied high-pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the Catalina Schist, a subduction zone metamorphic terrane. Migmatitic blocks contain leucocratic zones, veins, and pods with albitic plagioclase ? quartz ? muscovite. Mineralogically similar dikes and veins cut the ultra-mafic matrix; a few veins can be traced back into migmatitic blocks. Uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts. Field relations, mineral and bulk compositions, and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites. Element-partitioning, phase-equilibrium, and fluid-inclusion data are compatible with partial melting of the amphibolites at P = ∼8–11 kbar and T = ∼640–750 °C, in the presence of a low-salinity aqueous fluid. Although the Catalina metamorphism occurred at much shallower levels than those inferred for subduction-related magmatism, the process envisioned is one of multistage, slab-derived metasomatic contamination of the mantle in the “hanging wall” of a subduction zone.

Th-Pb ion-microprobe dating of allanite

Abstract Allanite, which is a common accessory mineral in a wide variety of rock types, typically contains high concentrations of Th and U; thus, an in-situ method of U-Th-Pb dating of this phase would have broad application. We describe a method to permit Th-Pb ages of allanite to be determined with approximately ±10% accuracy using a high-resolution ion microprobe. Knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ionization efficiencies of Th+ and Pb+. The chemical compositions of three allanite samples used as age standards (Cima d’Asta Pluton, 275.5 ± 1.5 Ma; Atesina Volcanic Complex, 276.3 ± 2.2 Ma; La Posta Pluton, 94 ± 2 Ma) were determined using an electron microprobe, permitting an assessment of matrix effects on ionization. An ion-microprobe calibration curve involving elemental and oxide species of Th and Pb (i.e., 208Pb*/Th+ vs. ThO2+/Th+) yields highly scattered apparent ages when allanite age standards with different Fe contents are used. However, a three-dimensional plot of 208Pb*/Th+ vs. ThO2+/Th+ vs. FeO+/SiO+ improves the accuracy of the calibration to about ±10%. Even though this level of uncertainty is substantially greater than that expected for U-Th-Pb ionmicroprobe analyses of zircon or monazite, Th-Pb ages of allanite can still be used to address important geologic questions. We used this method to date two metamorphic allanite grains from the footwall of the Main Central Thrust, Nepal Himalaya, and an allanite grain from the Pacoima Canyon pegmatite, California. Allanite inclusions in garnet from Nepal yield significantly older ages than the coexisting monazite, indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault. The Pacoima Canyon allanite grain yields a younger age than that reported for zircon, implying Pb loss during cooling of the pegmatite.

Primary fluids in low-temperature eclogites: evidence from two subduction complexes (Dominican Republic, and California, USA)

Eclogites occur as isolated blocks in melanges of both the Samana Peninsula, Dominican Republic, and the Franciscan Complex, California, USA. In some of these eclogites, fluid inclusions were found in omphacite and sodic-calcic amphibole grains. Textures show that non-planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole. In addition, planar arrays of secondary fluid inclusions are found along healed cracks. Homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions. These data were compared with petrologic geothermobarometry. Temperature conditions of 500–700° C were estimated from garnetclinopyroxene geothermometry. The jadeite contents of omphacite indicate minimum pressures of 8–11 kbar in this temperature range. The P-T estimates agree well with calculated isochores for primary fluid inclusions from the Samana Peninsula, and show some overlap for both primary and secondary fluid inclusions from the Franciscan Complex. Salinities of 1.2–5.3 wt% NaCl equiv. were estimated for both primary and secondary fluid inclusions from Samana and Franciscan eclogites. These data suggest that low-salinity aqueous fluids attended eclogite-facies metamorphism and perhaps retrograde metamorphism in both subduction complexes. The salinities and densities of fluid inclusions in eclogites from the Samana Peninsula and the Franciscan Complex resemble those of counterparts from garnet amphibolites of the Catalina Schist, southern California. An external source for such fluids is suggested by their homogeneous populations coupled with their low salinities. Geologic evidence suggests that the Samana and Franciscan eclogites may have been derived from a Catalina-like source terrane. The Catalina rocks are inferred to have interacted with large volumes of sediment-derived fluid during subduction zone metamorphism at similar P but higher T conditions than those determined for Samana and Franciscan eclogite blocks. These results contrast with data for fluid inclusions from eclogites of the Monviso area, western Alps. The Monviso eclogites yield similar estimates for metamorphic P-T to those obtained in this study, but contain fluid inclusions of brine and of other saline aqueous fluids, all of which are less dense than expected for incorporation at the reported eclogite-facies conditions. The differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the Samana-Franciscan-Catalina subduction complexes and those of Monviso probably reflect differences between fluid-flow regimes during metamorphism.

The origin of jadeitite-forming subduction-zone fluids: CL-guided SIMS oxygen-isotope and trace-element evidence

Abstract Jadeitite, a rare high P/T rock, is associated spatially with blueschist and/or eclogite terranes. Scanning electron microscope (SEM) and cathodoluminescence (CL) petrography of jadeitite samples from several major occurrences [in Burma (Myanmar), Guatemala, Japan, Kazakhstan, and the U.S.A.] show that grains were deposited from fluids. Jadeite grain compositions indicate these fluid compositions changed with time. CL imagery guided the acquisition of oxygen-isotope and trace-element analyses with the ion microprobe. Jadeite grains in each rock grew in cycles that began with red- and/or blue-luminescent and ended with green-luminescent zones. The CL images were used to order the data into crystallization sequences. These data and electron-microprobe, major-element analyses document the association of green CL with increases in Ca, Mg, and Cr: (1) toward grain exteriors; (2) in fine-grained matrix around porphyroblasts; (3) in shear zones that cut grains; (4) in former open spaces now filled with jadeite; or (5) in veins. Abundances of many trace elements are greater in green-CL jadeitite compared with the red- or blue-CL zones. Some of these elements.in particular Li, Rb, Sr, Ti, Hf, Zr, Y, and REE.are unlikely to have been derived from serpentinite. Although crystal-chemical effects may explain some of the trace-element systematics (e.g., preferential incorporation of REE into Ca-richer jadeite), some kinetic control is suggested by sector-zoned, rhythmically zoned grains. The oxygen-isotope data suggest that jadeitite-depositing fluids either had multiple sources or evolved in composition along their flow paths (or both).

Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)

Abstract The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower- T and - P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes. The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high- T melange of metamafic blocks in a matrix of meta-ultramafic rocks. This melange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both melanges are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments. Data from the Gee Point and Catalina melanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.

Metasomatism in a subduction complex: Constraints from microanalysis of trace elements in minerals from garnet amphibolite from the Catalina Schist

Trace element abundances and zoning were measured in minerals from a metasomatized garnet-amphibolite block from the Catalina Schist, using both ion and proton microprobes. Zoisite strongly concentrates light rare earth elements (REEs), Sr, Y, and Pb; amphibole concentrates Ni and Zn; garnet concentrates Y and heavy REEs; and titanite concentrates Nb. Major and trace elements in the garnets are zoned. Garnets in the core of the block display an overgrowth enriched in Mn, Y, and heavy REE, on a Y- and heavy-REE-poor core. Zirconium values remain relatively constant. The element enrichments in the garnet overgrowth suggest mobility of REEs at either a hand-sample or regional scale at subduction-zone pressure-temperature conditions. Metamorphic fluids may selectively transport heavy REEs relative to some high field strength elements in some convergent-margin settings. The distribution of Sr and Pb within subduction zones may reflect the dehydration and melting behavior of epidote-group minerals.

Jadeitite from Guatemala: new observations and distinctions among multiple occurrences

In Guatemala, jadeitite occurs as blocks in serpentinite melange in distinct settings on opposite sides of the Motagua fault. Jadeitites north of the Motagua fault are associated with eclogites, blueschists, and garnet amphibolites and distributed over a 200km E-W area. Omphacitite, omphacite - taramite metabasite, albitite, and phengite rock are found with jadeitite. The assemblages indicate formation at 6-12kbar and 300-400°C, however jadeite - omphacite pairs yield T from ~200 to >500oC for jadeite crystallization. Jadeitites south of the Motagua fault are sourced from three separate fault slices of serpentinite in Jalapa and Zacapa departments and are distinctive: 1) Jadeitite near Carrizal Grande is found in serpentinite with lawsonite eclogites, variably altered to blueschist, and rarely in schists. A large jadeite � omphacite gap and lawsonite suggests T=300-400°C, but at high P as indicated by the presence of quartz: P>12-20kbar. Lawsonite eclogites (P=20-25kbar, T=350-450°C) occur with these jadeitites. 2) At La Ceiba, jadeitites coexist with omphacite blueschists and contain late-stage veins of quartz, diopside, cymrite, actinolite, titanite and vesuvianite. A large jadeite � omphacite gap suggests 300-400°C, but at lower P as indicated by quartz + albite: P=10-14kbar. 3) At La Ensenada jade i tites occur with lawsonite-glaucophane blueschists and chloritite. It is a fine-grained jadeite-pumpellyite rock, intensely deformed and veined with grossular, omphacite, albite and titanite, but no quartz. A large jadeite�omphacite gap and pumpellyite suggest ~200-~300°C at lower P consistent with primary albite: P=6-9kbar. The silicates contain little iron.

Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California

For ∼200 km along the eastern Sierra Nevada continental magmatic arc, Mesozoic metavolcanogenic rocks dip steeply to the southwest (∼80°), a feature that must reflect fundamental processes in magmatic-arc construction. Although tight folds can account for such steep bedding tilts, folds in the metavolcanogenic sections are sparse and small scale. We propose that the high bedding tilts were produced by a combination of thrusting, downward displacement, and ductile deformation of the beds. The last two processes accompanied emplacement of the Sierra Nevada batholith. The Ritter Range pendant lies within this ∼200 km belt and provides a relatively large and well exposed Mesozoic volcanic section ranging in age from Late Triassic to mid–Cretaceous. Detailed mapping and ages from U-Pb zircon dates and fossils within the volcanic section reveal five structural blocks (I–V) that are separated by bedding-parallel thrusts, some of which are cryptic. To explain the present difference in bedding orientations between blocks III and IV, we suggest that the thrusting may have had a duplex geometry, which produced a maximum bedding dip of ∼45° in some blocks. Downward displacement of wall rock and ductile strain account for the remaining ∼35° of the observed average bedding dip (∼80°SW). The exact time of thrusting and duplex formation of Late Triassic to Early Jurassic rocks in blocks I–IV is uncertain, but these structures developed either (1) between 105 and 164 Ma, well before the other rotational processes were active, or (2) mostly around 105 Ma, and closer to the time when other rotational processes were active. Much of the subsequent (ca. 91–76 Ma) bedding tilting is related to downward displacement of beds associated with the emplacement of voluminous Late Cretaceous plutons, and to regional ductile deformation of the wall rocks during that period: the majority of the tilting probably took place between ca. 91 and 86 Ma. Bedding tilts of early to mid-Cretaceous rocks in blocks IV and V is bracketed between ca. 98 and ca. 90 Ma. Comparisons with metavolcanic sections to the northwest near Tioga Pass and to the southeast in the Mount Morrison, Mount Goddard and Oak Creek pendants, suggest that bedding rotation by thrusting(?), downward displacement and ductile strain of wall rock may explain the steep dips along this entire ∼200 km segment of the continental arc. Similar mechanisms may operate at midcrustal levels in other continental arcs.